51Nod 1649 齐头并进

 #include <iostream>
 #include <algorithm>
 #include <cstring>
 //两遍迪杰斯特拉
 #define INF 0xfffffff
 using namespace std;
 const int maxn =  + ;
 int rail[maxn][maxn];
 int dis[maxn];
 int vis[maxn];

 void dij(int n, int u){
     int i, j, p, Min;
     memset(vis, , sizeof(vis));
     for (int i = ; i <= n; i++)
         dis[i] = rail[u][i];
     vis[u] = ;
     dis[u] = ;

     for (i = ; i < n; i++){
         Min = INF;
         for (j = ; j <= n; j++){
             if (!vis[j] && dis[j] < Min){
                 p = j;
                 Min = dis[j];
             }
         }
         if (Min == INF) return;
         vis[p] = ;
         for (j = ; j <= n; j++){
             if (!vis[j] && dis[p] + rail[p][j] < dis[j]){
                 dis[j] = dis[p] + rail[p][j];
             }
         }
     }
 }

 int main(){
     ios::sync_with_stdio(false);
     fill(rail[], rail[] + maxn*maxn, INF);
     int n, m;
     cin >> n >> m;
     if (m == ){
         //没有铁路
         cout << "-1" << endl;
         return ;
     }
     for (int i = ; i < m; i++){
         int x, y;
         cin >> x >> y;
         rail[x][y] = ;
         rail[y][x] = ;
     }
     dij(n, );
     int ans1 = dis[n];
     if (ans1 == INF || m == (n*(n - ) / )){
         //铁路没有答案，或者没有公路
         cout << "-1" << endl;
         return ;
     }

     for (int i = ; i <= n; i++){
         for (int j =  + i; j <= n; j++){
             if (rail[i][j] == ){
                 rail[i][j] = INF;
                 rail[j][i] = INF;
             }
             else{
                 rail[i][j] = ;
                 rail[j][i] = ;
             }
         }
     }
     dij(n, );
     int ans2 = dis[n];
     int ans = ans1 > ans2 ? ans1 : ans2;
     cout << ans << endl;

     system("pause");
     return ;
 }